A vinyl alcohol polymer (hereinafter, may be abbreviated as “PVA”) has superior film-forming properties, interface characteristics and strength characteristics as one of few crystalline water soluble polymers. Therefore, PVA has been widely utilized as a thickening agent, a coating agent for paper, an adhesive, a processing agent for fibers, a binder, an emulsion stabilizer, a material of films and fibers, and the like. Furthermore, in order to improve specific performances of PVA, development of modified PVAs has been carried out by way of controlling crystallinity, introduction of a functional group and the like. Furthermore, a PVA-boric acid crosslinking solution in which a crosslinking reaction is caused by adding boric acid to PVA or the like has a property to gelate at low temperatures; therefore, the PVA-boric acid crosslinking solution is used for the purpose of controlling shape retention performances, or enhancing the strength of PVA matrices. However, for reasons such as toxicity of boron, and the like, restriction of the amount of boric acid or the like which may be used has started in recent years. Thus, a novel PVA or a PVA solution is demanded, which can be an alternative to PVA-boric acid crosslinking solutions, and the viscosity of which depending on the temperature can be controlled.
In known methods for increasing the viscosity of an aqueous PVA solution, an alkyl-modified PVA having an alkyl group introduced thereto is used. Such an alkyl-modified PVA is useful as a thickening agent for paints or adhesives since an interaction of alkyl groups (hydrophobic groups) occurs in water-based solvents to give a solution having high viscosity, and thus alkyl-modified PVAs having various types of monomer unit were also developed (see Japanese Unexamined Patent Application, Publication Nos. 2008-291120 and H10-338714). Although aqueous solutions of these alkyl-modified PVAs have a high viscosity at normal temperatures, the solutions have a high viscosity also at high temperatures, leading to inferior handleability; therefore, disadvantages of difficulties in preparation to give a homogeneous state, in particular, difficulties in preparation of an aqueous solution having a high concentration, and the like have been indicated.